Wallach's Rule Enforced by Pressure in Mandelic Acid - The Journal of

Jan 27, 2014 - It is a general thermodynamic rule that pressure-induced phase ... Citation data is made available by participants in Crossref's Cited-...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/JPCC

Wallach’s Rule Enforced by Pressure in Mandelic Acid Jędrzej Marciniak, Michał Andrzejewski, Weizhao Cai, and Andrzej Katrusiak* Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznan, Poland S Supporting Information *

ABSTRACT: At normal conditions, mandelic acid (MA) is exempt from Wallach’s rule, as enantiomers L-MA and D-MA are denser than racemate DL-MA. However, the enantiomers are less compressible than the racemate, DL-MA, which additionally is anomalously compressed at the phase transition at 0.65 GPa and becomes more dense than the enantiomers. Our results exclude the possibility of resolving racemic DL-MA into enantiomers due to their higher density in a spontaneous crystallization induced by pressure up to 2.6 GPa at least. Also, the L-MA crystal undergoes an isostructural phase transition at 1.52 GPa, allowing a tighter packing of molecules mimicking a racemic arrangement. It is a general thermodynamic rule that pressure-induced phase transitions in racemates and enantiomers have the opposite effect for their density relations and act toward or against Wallach’s rule, respectively.

■

INTRODUCTION Enantiomer resolution and asymmetric synthesis are of great importance for the pharmaceutical, chemical, and food industries. Four out of the five best-selling prescription drugs in the USA are chiral with estimated sales of more than 5.3 billion dollars in the first quarter of 2013. Enantiomers can differ in nutrition properties. For example, (−)-glucose tastes sweet, but it is not digested,1 which is considered in obesity therapy. Carvone (−) and (+) enantiomers have different scents,2 and racemate and enantiomers of menthol taste differently. Differences in enantiomers may concern their biological activity. Neurosteroid pregnenolone sulfate enantiomers differ ten times in enhancing spatial working memory in rats and mice.3 Teratogenic S-thalidomide contained in sedative medicines prescribed to expecting mothers had tragic consequences in the 1960s.4 Also carcinogenic activity can be associated with absolute configuration.5 On the other hand, the R-enantiomer of the drug Etodolac inhibits tumor development.6 For these and other reasons new methods of resolving enantiomers are continually developed. Jacques, Collet, and Wilen postulated high-pressure enantiomeric resolution of the chiral compounds defying Wallach’s rule.7 They estimated that the racemates less dense than the enantiomers should spontaneously separate below 1 GPa. According to Wallach’s rule,8,9 most of the racemic crystals are denser than the enantiomers. However, so far all attempts to enforce by pressure the spontaneous crystallization of racemates instead of conglomerates carried out for (±)-trans-1,2-diaminocyclohexane (DACH),10 (±)-2-butanol and (±)-2,3-butanediol11 failed. Exempt from Wallach’s rule is mandelic acid (MA, Figure 1), widely known as a urinary antiseptic,12 bacteriostatic and antiaging agent used in dermatology;13 MA esters are used in ophthalmology and in arteriosclerosis treatment. They are also employed as a small-molecule “switch” in allosteric recognition supramolecular systems.14 Racemate DL-MA, of orthorhombic space group Pbca (DL-MA I), is 0.054 g cm−3 less dense than © 2014 American Chemical Society

Figure 1. Independent molecules of L-MA crystal at 0.1 MPa and a pseudo glide plane a (green dots) perpendicular to [010].

enantiomers L-MA and D-MA.15−17 Racemic DL-MA occurs also as polymorph II, metastable at normal conditions. DL-MA II is monoclinic, with space group P21/c, and slightly denser than the enantiomers.18 Rietveld et al.19 interpreted a DSC signal splitting at 0.65 GPa and at 460 K as the pressure-induced resolution of DL-MA into enantiomers. However, this pressure value coincides with the transition between DL-MA phases I and II, which was evidenced by X-ray diffraction.20 Although the compression and structures of DL-MA phases have been measured and the pressure relation between DL-MA polymorphs I and II has been established, the compressibility of the enantiomer remained unknown, hence our study on the L-MA compression, aimed at obtaining the pressure dependence of racemates and enantiomers, which to our knowledge has not been reported for any substance until now.

■

EXPERIMENTAL METHODS (Sigma-Aldrich) single crystals were in situ grown at high pressure (Figure 2). High pressure was generated in a modified Merrill−Bassett diamond-anvil cell (DAC).21 Pressure was calibrated by the ruby-fluorescence method. 22,23 A L-MA

Received: November 29, 2013 Revised: January 3, 2014 Published: January 27, 2014 4309

dx.doi.org/10.1021/jp411738p | J. Phys. Chem. C 2014, 118, 4309−4313

The Journal of Physical Chemistry C

Article

Figure 2. Isochoric growth of L-MA from acetone solution at (a) 355 K, (b) 320 K, and (c) 296 K/0.3 GPa. Ruby chip used for pressure calibration lies above the L-MA crystal. Miller indices of selected faces are given in brackets (c).

Figure 3. Isothermal compression of a L-MA single crystal (in situ grown isochorically in the DAC) through the transition from (a) phase I at 1.4 GPa/296 K to (b) phase II at 1.62 GPa/296 K and back to (c) phase I at 1.48 GPa/296 K. A ruby chip for pressure calibration lies by the top edge of the gasket.

PhotonControl spectrometer with enhanced resolution was used, affording an accuracy of 0.02 GPa. The L-MA crystal samples were loaded into the chamber, 0.4 mm in diameter and 0.2 mm high, in a preindented steel gasket. To explore L-MA high-pressure behavior, the single crystals were not only isothermally compressed but also dissolved and in situ isochorically recrystallized in the DAC at high pressure (Figure 2). We tested a series of solvents to find optimal recrystallization conditions, and acetone proved to be the best solvent for this purpose. For the experiments above the hydrostatic limit of acetone, 0.93 GPa according to Bridgman,24a we used isopropanol as the solvent and pressure medium (liquid up to 4.2 GPa at 296 K).24b Isochoric crystallizations at 0.3, 0.6, 0.9 and 1.27 GPa were followed by isothermal sample compression to 1.4, 1.52, 1.55, 1.79, 2.3, and 2.6 GPa. We chose this pressure range to compare the highpressure behavior of L-MA with DL-MA, previously studied up to 1.36 GPa. This pressure range was fully sufficient for the Wallach’s rule investigation. At each of these pressure points the lattice dimensions and crystal structure were determined by single-crystal X-ray diffraction. We have established that at 1.52 GPa the L-MA crystal undergoes an enantiotropic and isostructural transition to a new phase (Table 1). The isostructural phase transitions preserve the space-group symmetry of crystals (including the approximate unit-cell dimensions) but can change their physical properties.25−29

The diffraction data were collected by an Xcalibur Eos diffractometer with Mo Kα X-rays. Previously described procedures for the DAC alignment and the data collections were applied. 30 CrysAlis software 31 controlling the diffractometer was used for the initial data reduction. The structures were refined with full-matrix least-squares on F2’s in Shelxl.32,33 Hydrogen atoms were ideally positioned according to the molecular geometry.

■

RESULTS AND DISCUSSION High-pressure in situ crystallizations showed that racemate DLMA remains stable up to 1.36 GPa at least: phase DL-MA I from 0.1 to 700 MPa and phase DL-MA II at higher pressure (Figure 4).20 According to our present X-ray diffraction measurements,

Table 1. Selected Crystal Data of Mandelic Acid Forms, at Their Lowest Limits of the Pressure Stability Regions (Pmin) DL-MA

Pmin space group a [Å] b [Å] c [Å] β [°] Vm [Å3] Z Dx [g cm−3]

I

0.1 MPa Pbca 9.669(2) 16.183(3) 9.953(2) 90 194.67 8 1.298

DL-MA

II

0.76 GPa P21/c 5.825(2) 28.908(11) 8.224(6) 93.03(4) 172.86 8 1.462

L-MA

I

0.1 MPa P21 8.6199(8) 5.8602(4) 15.1701(12) 102.785(8) 186.83 4 1.352

L-MA

II

1.55 GPa P21 7.7084(14) 5.7868(8) 14.79(3) 96.38(4) 163.95 4 1.541

Figure 4. Density of MA solid phases as a function of pressure. Phase transitions in DL-MA and in L-MA are indicated by vertical dashed lines. The DL-MA structure determinations in phases I and II20 are marked by blue and red circles; those of the L-MA phase I are shown as black and L-MA phase II as green squares. Empty symbols refer to the low-temperature crystal measurements. L-MA is compressed at a smaller rate than DL-MA I between 0.1 and 700 MPa. The transition between phases DL-MA I and II is associated with a considerable molecular volume drop ΔVm of 9.15 Å3. Due to this ΔVm, at 700 MPa the racemate becomes more dense than the enantiomer L-MA. Thus above the phase transition in DL-MA at 700 MPa the Wallach’s rule becomes valid for MA. The phase transitions in both DL-MA and L-MA do not significantly change the compressibility of these compounds (Figure 4), and the racemate continues to be softer than the enantiomer. Therefore, the extrapolation of DL-

The phase transition between phases I and II of L-MA is reversible. The L-MA single crystal survives this transition both on increasing and on decreasing pressure (Figure 3). It is characteristic of isostructural phase transitions that they are discontinuous and first-order in character according to the Ehrenfest’s classification. The L-MA phases below and above 1.52 GPa were denoted as phases I and II, respectively. 4310

dx.doi.org/10.1021/jp411738p | J. Phys. Chem. C 2014, 118, 4309−4313

The Journal of Physical Chemistry C

Article

between phenyl rings of molecules A and B abruptly decreases after the transition between phases L-MA I and II and gradually decreases to 4.03° at 2.3 GPa. This is due to the tighter packing of phenyl rings in the pseudosymmetric environment. In phase L-MA I the departure from the pseudocentrosymmetric structure is geared to the chiral centers C2 and C10. These centers generate small voids around phenyl rings in the pseudocentrosymmetric structure. When pressure eliminates these voids, the rings are more parallel, and the pseudosymmetry increases. This is a likely reason of conformational changes of the L-MA molecules (Figure 6).

MA II data above 1.36 GPa strongly suggests that it remains more dense than L-MA to 1.6 GPa at least. The high-pressure racemate DL-MA II can exist also at ambient pressure as a metastable phase. It is remarkable that its density is much higher than that of DL-MA I and even marginally higher than the density of enantiomer L-MA. Thus, with respect to racemate DL-MA II, the Wallach’s rule is valid in all pressure ranges investigated. However, at ambient conditions the metastable phase DL-MA II transforms to phase DL-MA I after a few days, which is one of the examples that more dense phases can be metastable at ambient pressure. In compressed L-MA at 1.52 GPa there is a clear volume drop ΔVm of 2.4 Å3, associated with a small discontinuous reduction of the unit-cell dimensions, somewhat counteracted by a considerable change of 5° in the monoclinic angle β (Figures 4 and 5).

Figure 6. Torsion angles in the molecules of DL-MA I (blue), DL-MA II (red), L-MA phase I (black), L-MA II (green), as well as the dihedral angle between symmetry-independent phenyl rings in L-MA. Phase transitions in DL-MA and in L-MA are indicated by vertical dashed lines. Figure 5. Unit-cell compression of L-MA phases I and II. Empty symbols refer to the low-temperature parameters. Angle β is plotted vs pressure in the inset. The transition pressure in L-MA is indicated by the vertical dashed line.

The space-group symmetry of the crystal below and above 1.52 GPa remains the same, which is characteristic of isostructural phase transitions. The structures of polymorphs L-MA I below 1.52 GPa and L-MA II above this pressure are very similar not only in the lattice parameters and crystal symmetry but also in the molecular positions. The extent of changes in these positions at 1.52 GPa is so small that the structure of phase L-MA II could be refined with the starting model of phase L-MA I. There are two symmetry-independent molecules in the L-MA phases I and II. The most apparent structural change at 1.52 GPa is in their conformation and in the dihedral angle between best planes fitted to the aromatic rings (Figure 5 and Figure S4, Supporting Information), whereas the positions of OH···O bonded groups are less affected. Interestingly, the crystal structure of L-MA “mimics” in its certain features the centrosymmetric space group P21/a; it can be seen in projection down [010] in Figure 7 that the symmetry-independent molecules A and B in L-MA are approximately related by a pseudocenter of inversion and a pseudo glide plane a (Figure 1). The ideal inversion center between molecules would require their phenyl rings to be parallel. It can be seen in Figure 7 that indeed the angle

Figure 7. Molecular arrangement in the crystals of: (a) L-MA I, space group P21, and (b) DL-MA II, space group P21/c at 0.1 MPa. In L-MA (a) the symmetry-independent molecules B are shaded blue. The pseudosymmetry elements in L-MA I are shown in green (a).

It can be concluded that pressure reverses the density relation between enantiomers and racemates of mandelic acid. This reversed relation is the combined effect of the weaker compression of the enantiomers and of the first-order phase transition in DL-MA. Consequently, MA obeys the Wallach’s rule above 0.7 GPa. Because the racemate becomes more dense, its postulated resolution to the enantiomers by pressure19 is 4311

dx.doi.org/10.1021/jp411738p | J. Phys. Chem. C 2014, 118, 4309−4313

The Journal of Physical Chemistry C

Article

(5) Karle, I. L.; Yagi, H.; Sayer, J. M.; Jerina, D. M. Crystal and molecular structure of a benzo[a]pyrene 7,8-diol 9,10-epoxide N2deoxyguanosine adduct: Absolute configuration and conformation. Proc. Natl. Acad. Sci. U.S.A. 2004, 101 (6), 1433−1438. (6) Kolluri, S. K.; Corr, M.; James, Y. S.; Bernasconi, M.; Lu, D.; Liu, W.; Cottam, H. B.; Leoni, L. M.; Carson, D. A.; Zhang, X. The Renantiomer of the nonsteroidal antiinflammatory drug etodolac binds retinoid X receptor and induces tumor-selective apoptosis. Proc. Natl. Acad. Sci. U.S.A. 2005, 102 (7), 2525−2530. (7) Jacques, J.; Collet, A.; Wilen, S. H. Enantiomers, Racemates and Resolutions; Krieger Publishing Company: Malabar, FL, 1994. (8) Wallach, O. Zur Kenntniss der Terpene und der ätherischen Oele. Ueber gebromte Derivate der Carvonreihe. Liebigs Ann. Chem. 1895, 286, 90−143. (9) Brock, C. P.; Schweizer, W.; Dunitz, J. D. On the validity of Wallach’s rule: on the density and stability of racemic crystals compared with their chiral counterparts. J. Am. Chem. Soc. 1991, 113 (26), 9811−9820. (10) Cai, W.; Katrusiak, A. Enantiomeric Crystallization of (±)-trans1,2-Diaminocyclohexane under Pressure. CrystEngComm 2011, 13, 6742−6746. (11) Podsiadło, M.; Patyk, E.; Katrusiak, A. Chiral Aggregation Hierarchy in High-Pressure Resolved 2-Butanol and 2,3-Butanediol. CrystEngComm 2012, 14, 6419−6423. (12) Putten, P. L. Mandelic Acid and Urinary Tract Infections. Antonie van Leeuwenhoek 1979, 45, 622. (13) Taylor, M. Summary of Mandelic Acid for the Improvement of Skin Conditions. J. Cosmet. Dermatol. 1999, 21, 26−28. (14) Heo, J.; Mirkin, C. A. Pseudo-Allosteric Recognition of Mandelic Acid with an Enantioselective Coordination Complex. Angew. Chem. 2006, 45 (6), 941−944. (15) Rose, H. A. Crystallographic Data. 61. dl-Mandelic Acid. Anal. Chem. 1952, 24, 1680−1681. (16) Wei, K.-T.; Ward, D. L. α-Hydroxyphenylacetic Acid: a Redetermination. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1977, 33, 797−800. (17) Mughal, R. K.; Gillon, A. L.; Davey, R. J. DL-Mandelic Acid: CCDC 602882, 2006. (18) Fischer, A.; Profir, V. M. A Metastable Modification of (RS)Mandelic Acid. Acta Cryst. E 2003, 59, 1113−1116. (19) Rietveld, I. B.; Barrio, M.; Tamarit, J.-L.; Do, B.; Ceolin, R. Enantiomer Resolution by Pressure Increase: Inferences from Experimental and Topological Results for the Binary Enantiomer System (R)- and (S)-Mandelic Acid. J. Phys. Chem. B 2011, 115, 14698−14703. (20) Cai, W.; Marciniak, J.; Andrzejewski, M.; Katrusiak, A. Pressure Effect on D,L-Mandelic Acid Racemate Crystallization. J. Phys. Chem. C 2013, 117, 7279−7285. (21) Merrill, L.; Bassett, W. A. Miniature diamond anvil pressure cell for single crystal X-ray diffraction studies. Rev. Sci. Instrum. 1974, 45, 290−294. (22) Piermarini, G. J.; Block, S.; Barnett, J. D.; Forman, R. A. Calibration of the Pressure Dependence of the R1 Ruby Fluorescence Line to 195 kbar. J. Appl. Phys. 1975, 46, 2774−2780. (23) Mao, H. K.; Xu, J.; Bell, P. M. Calibration of the ruby pressure gauge to 800 kbar under quasi-hydrostatic conditions. J. Geophys. Res. 1986, 91, 4673−4676. (24) (a) Bridgmann, P. W. The physics of high-pressure; G. Bell and sons: London, 1931. (b) Angel, R. J.; Bujak, M.; Zhao, J.; Gatta, G. D.; Jacobsen, S. D. Effective hydrostatic limits of pressure media for highpressure crystallographic studies. J. Appl. Crystallogr. 2007, 40, 26−32. (25) Li, Q.; Li, S.; Wang, K.; Li, X.; Liu, J.; Liu, B.; Zou, G.; Zou, B. Pressure-induced isosymmetric phase transition in sulfamic acid: A combined Raman and x-ray diffraction study. J. Chem. Phys. 2013, 138, 214505. (26) Christy, A. G. Isosymmetric Structural Phase Transitions: Phenomenology and Examples. Acta Crystallogr., Sect. B 1995, 51, 753−757.

highly unlikely. The effect of the phase transition in racemate reduces the possibility of resolving the enantiomers. Phase transitions in enantiomers cannot be neglected, either, as evidenced by the pressure-induced transformation of L-MA at 1.52 GPa. It can be generally stated that all pressure-induced first-order phase transitions of any racemate act toward increasing the D(racemate)/D(enantiomer) density ratio. For example, the transition of DL-MA at 0.7 GPa increases this ratio from 0.960 to 1.007 and restores the Wallach’s rule. All pressure-induced first-order phase transitions in enantiomers reduce the D(racemate)/D(enantiomer) ratio. In L-MA, the transition at 1.52 GPa reduces this ratio from 1.026 to 1.013, but the ΔV is too small to violate Wallach’s rule again. It is also sufficient to know that no other first-order phase transitions occur in L-MA up to 2.6 GPa to the state that Wallach’s rule extends to this pressureany phase transition in DL-MA can only increase the density of the racemate. Until now, no pressure-induced transformation resolving racemate into conglomerate, nor vice versa, has been reported. Apart from MA, the compressibility information in both racemates and enantiomers is known on very few chiral compounds, to our knowledge amino acids, serine,34−38 and cysteine.39,40 All this information is consistent with the role of phase transitions for the crystal density and volume. The general rules describing the effect of phase transitions of racemates and enantiomers will considerably facilitate the further research on pressure-induced preferential crystallization of racemates and conglomerates.

■

ASSOCIATED CONTENT

S Supporting Information *

Molecular volume graphs, images of hydrogen bond nets, crystal data, and molecular conformation before and after the phase transition comparison. This material is available free of charge via the Internet at http://pubs.acs.org. The structural information have been deposited with the Cambridge Crystallographic Data Centre in the form of CIF files, numbers: 965992−966005.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +48(61)8291590. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This study was supported by the Foundation for Polish Science TEAM Grant 2009-4/6. REFERENCES

(1) Garrett, R.; Grisham, C. Biochemistry; Cengage Learning, Inc.: Boston, 2009. (2) Brookes, J. C.; Horsfield, A. P.; Stoneham, A. M. Odour character differences for enantiomers correlate with molecular flexibility. J. R. Soc. Interface 2009, 6 (30), 75−86. (3) Akwa, Y.; Ladurelle, N.; Covey, D. F.; Baulieu, E.-E. The synthetic enantiomer of pregnenolone sulfate is very active on memory in rats and mice, even more so than its physiological neurosteroid counterpart: Distinct mechanisms? Proc. Natl. Acad. Sci. U.S.A. 2001, 98 (24), 14033−14037. (4) Eriksson, T.; Bjöurkman, S.; Roth, B.; Fyge, Å.; Höuglund, P. Stereospecific Determination, Chiral Inversion in Vitro and Pharmacokinetics in Humans of the Enantiomers of Thalidomide. Chirality 1995, 7, 44−52. 4312

dx.doi.org/10.1021/jp411738p | J. Phys. Chem. C 2014, 118, 4309−4313

The Journal of Physical Chemistry C

Article

(27) Paliwoda, D.; Kowalska, K.; Hanfland, M.; Katrusiak, A. U-Turn Compression to a New Isostructural Ferrocene Phase. J. Phys., Lett. 2013, 4 (23), 4032−4037. (28) Patyk, E.; Skumiel, J.; Podsiadlo, M.; Katrusiak, A. HighPressure (+)-Sucrose Polymorph. Angew. Chem., Int. Ed. 2012, 51, 2146−2150. (29) Cai, W.; Katrusiak, A. Conformationally Assisted Negative Area Compression in Methyl Benzoate. J. Phys. Chem. C 2013, 117, 21460− 21465. (30) Budzianowski, A.; Katrusiak, A. In High-Pressure Crystallography; Katrusiak, A., McMillan, P. F., Eds.; Kluwer Academic Publishers: Dordrecht, 2004. (31) Xcalibur CCD System, CrysAlisPro Software System, version 1.171.33; Oxford Diffraction Ltd.: Wrocław, Poland, 2009. (32) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. OLEX2: a complete structure solution, refinement and analysis program. J. Appl. Crystallogr. 2009, 42, 339−341. (33) Sheldrick, G. M. A Short History of SHELX. Acta Crystallogr., Sect. A 2008, 64, 112−122. (34) Zakharov, B. A.; Kolesov, B. A.; Boldyreva, E. V. Effect of pressure on crystalline L- and DL-serine: revisited by a combined single-crystal X-ray diffraction at a laboratory source and polarized Raman spectroscopy study. Acta Crystallogr., Sect. B 2012, 68, 275− 286. (35) Moggach, S. A.; Allan, D. R.; Morrison, C. A.; Parsons, S.; Sawyer, L. Effect of pressure on the crystal structure of L-serine-I and the crystal structure of L-serine-II at 5.4 GPa. Acta Crystallogr., Sect. B 2005, 61, 58−68. (36) Boldyreva, E. V.; Sowa, H.; Seryotkin, Yu.V.; Drebushchak, T. N.; Ahsbahs, H.; Chernyshev, V.; Dmitriev, V. Pressure-induced phase transitions in crystalline l-serine studied by single-crystal and highresolution powder X-ray diffraction. Chem. Phys. Lett. 2006, 429, 474− 478. (37) Boldyreva, E. V.; Kolesnik, E. N.; Drebushchak, T. N.; Ahsbahs, H.; Beukes, J. A.; Weber, H.-P. A comparative study of the anisotropy of lattice strain induced in the crystals of L-serine by cooling down to 100 K or by increasing pressure up to 4.4 GPa. Z. Kristallogr. 2005, 220, 58−65. (38) Drebushchak, T. N.; Sowa, H.; Seryotkin, Yu.V.; Boldyreva, E. V.; Ahsbahs, H. L-Serine III at 8.0 GPa. Acta Crystallogr., Sect. E 2006, 62, 4052. (39) Moggach, S. A.; Allan, D. R.; Clark, S. J.; Gutmann, M. J.; Parsons, S.; Pulham, C. R.; Sawyer, L. High-pressure polymorphism in L-cysteine: the crystal structures of L-cysteine-III and L-cysteine-IV. Acta Crystallogr., Sect. B 2006, 62, 296−309. (40) Minkov, V. S.; Tumanov, N. A.; Cabrera, R. Q.; Boldyreva, E. V. Low temperature/high pressure polymorphism in DL-cysteine. CrystEngComm 2010, 12, 2551−2560.

4313

dx.doi.org/10.1021/jp411738p | J. Phys. Chem. C 2014, 118, 4309−4313